Sitagliptin, the chemical name of which is (2R)-4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4]triazol[4,3-α]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-amine, is a member of a class of antihyperglycemic agents called dipeptidyl peptidase 4 (DPP-IV) inhibitors which improve blood glucose control in type 2 diabetes patients by increasing the concentrations of incretin hormones. These hormones, including glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), are released by the intestine throughout the day and their concentrations increase in response to food intake. The activity of GLP-1 and of GIP is limited by the DPP-IV enzyme which quickly hydrolyzes incretins and transforms them into inactive products. Sitagliptin prevents this incretin hydrolysis by DPP-IV, so plasma concentrations of active forms of GLP-1 and GIP increase. As a result, sitagliptin increases insulin release and reduces the concentration of glucagon in a glucose dependent manner.

Sitagliptin was commercially authorized by the European Medicines Agency in March 2007. The medicinal product is currently marketed under the name Januvia®, being formulated with the dihydrogen phosphate salt of sitagliptin.
European patent EP 1412357 B1 (WO 2003/004498 A2) protects sitagliptin and its pharmaceutically acceptable salts. Likewise, in relation to the marketed medicinal product, a subsequent European patent EP 1654263 B1 (WO 2005/003135 A2) (MERCK; Jul. 18, 2004) specifically protects the dihydrogen phosphate salt of sitagliptin.
It is known by the person skilled in the art that different salts of one and the same pharmaceutically active substance can have different physical properties (melting point, solubility, hygroscopicity, etc). Even the existence of different crystalline forms of one and the same salt of one and the same pharmaceutically active substance can also have considerably different physical properties. Said differences can accordingly influence the pharmaceutical properties of the medicinal product formulated with the crystalline forms, without the person skilled in the art being able to predict a priori the existence of these crystalline forms or the different physical characteristics that they may have.
In this respect, several documents of the state of the art have described the existence of different crystalline forms of the addition salt of sitagliptin with sulfuric acid.
International application WO 2009/085990 A2 describes the production of an anhydrous crystalline form of sitagliptin sulfate salt and its characterization by means of DSC (Differential Scanning calorimetry), TGA (Thermal Gravimetric Analysis) and XRPD (X-Ray Powder Diffraction). However and in the hands of the inventors of the present application, said crystalline form is hygroscopic under the conditions of the European Pharmacopeia Ed. 6.0 Section 5.11 (24 hours at 80% RH and 25° C.), readily reaching a water proportion corresponding to a dihydrate form.
International application WO 2010/000469 A2 describes the production of an ethanol solvate (Form I) and of a crystalline form of sitagliptin sulfate salt (Form II). Both forms were characterized by means of DSC and XRPD, Form II having a water content corresponding to a monohydrate compound. This same form quickly reaches greater degrees of hydration (Table 21 of the application) under 43% and 75% relative humidity conditions, indicating the hygroscopic characteristics thereof.
International application WO 2010/092090 A2 describes the production of a crystalline form of sitagliptin sulfate salt, the characterization of which by means of XRPD is consistent with the crystalline form described in International application WO 2009/085990 A2.
Finally, international applications WO 2010/117738 A1 and WO 2011/123641 A1 describe the production of 19 crystalline forms (called S1 to S20 except form S15 which is not described) of sitagliptin sulfate salt (some of them, for example S4 and S5, being those previously described in the applications indicated above). The crystalline forms of these two applications are mainly characterized by means of their corresponding XRPD. According to the inventors, forms S7 and S13 are isopropanol solvate and methanol solvate, respectively. Crystalline form S14 is a monohydrate, crystalline form S16 is a sesquihydrate and crystalline forms S1, S9 and S11 are dihydrates. Despite not further describing physical properties of the remaining crystalline forms, there is data relating to same which indicate that crystalline forms S2 and S6 are transformed into form S9 under 100% relative humidity conditions, crystalline forms S13 and S16 and S18 are transformed into crystalline forms S14, S17 and S14, respectively, when the wet compound residual solvent is dried in a standard manner, crystalline form S17 is transformed into form S14 by being kept at a temperature of 50° C. (24 hours) or form S14 is transformed into form S1 or into a mixture of forms S1 and S11 when they are subjected to an environment with a high relative humidity. They also describe that under standard wet compound residual solvent drying conditions, crystalline form S11 is transformed into form S12, crystalline form S16 is transformed into crystalline form S17, which in turn is transformed into crystalline form S14, crystalline form S18 is transformed into crystalline form S14. Furthermore, patent application WO 2011/123641 A1 states that crystalline forms S7 and S10 filter slowly.
Based on what is described in the state of the art, it is necessary to obtain a crystalline form of the addition salt of sitagliptin with sulfuric acid with physical characteristics that allows it to be formulated in pharmaceutical compositions with assured stability.